Decade-to-century scale variability and change in the Earth System components

The physical Earth System consists of the atmosphere, the hydrosphere (oceans
and rivers), the cryosphere (sea- and land-ice and snow), and the biosphere
(land surface-vegetation). Interactions within and among these Earth System
components, forced by solar radiation, generates the Earth's climate. These
interactions and variations in the Earth System's radiative forcings produce
variations in climate state, characterized by precipitation, temperature, winds,
storminess, etc.. Timescales of such variations are determined by characteristic
properties of the Earth System components, such as thermal and mechanical inertia,
and adjustment timescales of each component's mass, momentum, and energy. Because
air is lighter and can store less heat per unit mass than land, ice, and water,
it has faster characteristic adjustment timescales than the others. Characteristic
adjustment timescales of the atmosphere and the oceans also depend on the Earth's
rotation speed and thus on latitude. Typical adjustment timescales are days
to weeks for the atmosphere, weeks to years for the biosphere, months to decades
for the ice, and months to centuries for the oceans. Therefore, perturbations
to the atmosphere can produce climate variability at days to weeks timescales
whereas perturbations to the slower Earth System components can produce climate
variability at months to centuries timescales.

Radiative forcings on and within the Earth System also vary. Solar radiation
at the top of the atmosphere is known to vary at years to millennia timescales;
variations or changes in atmospheric constituents, such as ozone, carbon dioxide,
methane, and aerosols, can also cause radiative forcings within the Earth System
to vary or change, possibly causing climate variations or climate change.

Known manifestations of societal impacts of decade-to-century scale climate
variations go back at least 2400 years before the present. Meton, a water resources
engineer in Athens around 400 B.C., speculated about the possible influence
of solar variability, as he observed in sunspot variability, on rainfall and,
consequently, on the Athenian water supply. In the 19th and 20th centuries,
a large body of literature developed on observed relationships between climate
variability in various parts of the world and solar and lunar influences primarily
at decade-to-century timescales. In the last two decades, as our knowledge of
the Earth System has expanded, other mechanisms of decade-to-century scale variability
have been proposed in which known attributes of Earth System componenst generate
such variability without or with accompanying variability in external forcings
on the Earth System.